1. Field of the Invention
The present invention relates generally to a silicone-coated polymeric film having a back side coating effective to reduce the undesirable slip generated by the by-product or contaminant silicone oil released from the silicone coating. It further relates to a method for controlling undesirable slip in silicone-coated polymeric film by coating the back side of the film with a slip control coating.
2. Description of Related Art
Silicone-coated polymer films are well known and commercially available. Such silicone-coated polymer films are commonly employed as release films for labels, pressure sensitive tapes, decorative laminates, transfer tapes and the like. The silicone coating on the base polymer film allows the adhesive face of the label, for example, to be easily removed from the support film by the end user, while providing sufficient stability to the label to prevent it from being accidentally dislodged from the support film before use. Among other things, silicone release coatings can also be used as container liners designed for contact with food. Pizza box liners coated with silicone release coatings, for example, allow melted cheese to release easily. Food waste is minimized, the food is served more easily, and the appearance of the product is maintained.
A variety of different silicone release coatings are known in the art. These silicone release coatings include crosslinked formulations, ultraviolet curable or heat curable formulations, solvent-based and solvent-free formulations, and combinations of the foregoing, such as solvent-free, ultraviolet curable formulations. As disclosed in U.S. Pat. No 5,350,601 to Culbertson et al., incorporated herein by reference in its entirety, primer formulations have been discovered that allow these silicone release coatings to bond effectively with the base polymer films, and to remain bonded over time to the base polymer film. These primer formulations are typically applied to the base polymer film, and the silicone release coating is applied on top of the resulting primer layer. The resulting coated polymer film provides superior performance. For example, coating rub-off, which can cause labels to release too easily, is minimized. Moreover, coating delamination, which can cause labels to adhere directly to the underlying base polymer film, is also minimized or eliminated.
However, during manufacture and during storage of the coated film prior to use, free silicone oil is inevitably released from the coating layer onto the surface of the coated film. This migration creates a high slip layer or discrete patches of free silicone oil on the film surface. Free silicone oil can also be transferred easily to the non-silicone-coated back side of the film. When, for example, the coated film is stored in front-to-back-face contact, as in a typical roll of film, some of the free silicone oil (typically low molecular weight silicones) on the front, silicone coated surface of the film will be transferred to the back (typically uncoated) surface of the roll. In the film roll, which is typically formed during manufacture for storage or transport, the front, silicone coated surface is placed in contact with the back surface of the same web as it is wound.
In practice, this free silicone oil contamination can be a drawback. For example, when the coated film traverses over idler rolls during subsequent processing steps (e.g., during label application), the idler rolls can slip and lose contact with the film. This can result in uneven tension on the film web or loss of alignment. Related problems arise when printing the resulting silicone-coated film or labels applied to that film. The ability of printing inks and solvents to adhere is impaired by the presence of free silicone oil on the surface to be printed. Moreover, loss of alignment due to the film's high slip can lead to a high reject rate during printing, particularly where multiple printing passes are used and proper alignment is crucial to forming an integrated final image.
The prior art has recognized the existence of the free silicone oil on the surfaces of the coated film, and the resulting transfer contamination problem. However, no suggestion has been made in the known art of any approach to controlling the free silicone oil generated by the coated film. Instead, in an implicit acknowledgment that no solution to the transfer contamination problem was available or evident, prior art patents (e.g., U.S. Pat. No. 5,110,671 to Balloni et al. and U.S. Pat. No. 4,961,992 to Balloni et al.) have attempted to use the free silicone oil and the resulting increased slip to improve processing.
Nonetheless, it remains commercially desirable to control the effects of free silicone oil. Film processing parameters are highly sensitive to changes in film friction properties. Ideally, silicone-coated films would have a coefficient of friction ("COF") equivalent to that of plain film to simplify processing. This would eliminate the costly and time consuming equipment and setting modifications that are required to process film having substantially different COFs, as well as the higher reject rate inherent in processing under significantly different COF conditions.
U.S. Pat. No. 5,350,601 to Culbertson et al., discussed above, discloses a silicone-ready primer coating including a glycidoxy silane and a copolyester. This primer coating is designed for use under a silicone coating, to improve the integrity of the silicone coating on a base polymer film. While the Culbertson et al. patent makes reference to coating both sides of the film with the primer coating, it is envisioned only that a silicone coating will be applied to all primer coated surfaces. There is no suggestion that one side would be coated while the other would be left uncoated. More importantly, there is no suggestion in the Culbertson et al. patent of the silicone-binding properties of the silane and copolyester coating.
A commercial product sold at least as early as one year prior to the filing date of the present application by the assignee of the present invention under the designation Hostaphan.RTM. 3SAC polyester film included a silicone-ready primer coating on both sides. That silicone-ready primer coating included an isophthalic acid/sodium 5-sulfoisophthalic acid/malonic acid/ethylene glycol copolyester, a glycidoxypropyltrimethyoxysilane, a colloidal SiO.sub.2 and sodium lauryl sulfate. However, neither side of the film, as sold by the assignee, was silicone coated. The film was sold as a silicone-ready film, and purchasers of the film are known to have coated it with silicone. However, the inventors are not aware of any purchaser who coated one side with silicone, but not the other. Alternate films with the silicone-ready coating on a single side only (e.g., 3SAB polyester film) were available to customers not interested in silicone coating both sides.